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Prerequisites for
Configuring SSM

The
following are the prerequisites for configuring source-specific multicast (SSM)
and SSM mapping:

Before you configure SSM
mapping, you must perform the following tasks:

Before you configure static
SSM mapping, you must configure access control lists (ACLs) that define the
group ranges to be mapped to source addresses.

Before you can configure and
use SSM mapping with DNS look ups, you must be able to add records to a running
DNS server. If you do not already have a DNS server running, you need to
install one.

Note

You can use
a product such as
Cisco Network Registrar
to add records to a running DNS server.

Restrictions for Configuring SSM

The following are the restrictions for configuring SSM:

To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application is running, and the
application itself.

The SSM mapping feature does not have all the benefits of full SSM. Because SSM mapping takes a group join from a host and
identifies this group with an application associated with one or more sources, it can only support one such application per
group. Full SSM applications can still share the same group as in SSM mapping.

Enable IGMPv3 carefully on the last hop router when you rely solely on SSM mapping as a transition solution for full SSM.
When you enable both SSM mapping and IGMPv3 and the hosts already support IGMPv3 (but not SSM), the hosts send IGMPv3 group
reports. SSM mapping does not support these IGMPv3 group reports, and the router does not correctly associate sources with
these reports.

Existing applications in a network predating SSM do not work within the SSM range unless they are modified to support (S,
G) channel subscriptions. Therefore, enabling SSM in a network can cause problems for existing applications if they use addresses
within the designated SSM range.

Address management is still necessary to some degree when SSM is used with Layer 2 switching mechanisms. Cisco Group Management
Protocol (CGMP), IGMP snooping, or Router-Port Group Management Protocol (RGMP) support only group-specific filtering, not
(S, G) channel-specific filtering. If different receivers in a switched network request different (S, G) channels sharing
the same group, they do not benefit from these existing mechanisms. Instead, both receivers receive all (S, G) channel traffic
and filter out the unwanted traffic on input.

Because SSM can re-use the group addresses in the SSM range for many independent applications, this situation can lead to
decreased traffic filtering in a switched network. For this reason, it is important to use random IP addresses from the SSM
range for an application to minimize the chance for re-use of a single address within the SSM range between different applications.
For example, an application service providing a set of television channels should, even with SSM, use a different group for
each television (S, G) channel. This setup guarantees that multiple receivers to different channels within the same application
service never experience traffic aliasing in networks that include Layer 2 switches.

In PIM-SSM, the last hop router continues to periodically send (S, G) join messages if appropriate (S, G) subscriptions are
on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the shortest path tree (SPT) state from the
receivers to the source is maintained, even if the source does not send traffic for longer periods of time (or even never).

The opposite situation occurs with PIM-SM, where (S, G) state is maintained only if the source is sending traffic and receivers
are joining the group. If a source stops sending traffic for more than 3 minutes in PIM-SM, the (S, G) state is deleted and
only reestablished after packets from the source arrive again through the RPT (rendezvous point tree). Because no mechanism
in PIM-SSM notifies a receiver that a source is active, the network must maintain the (S, G) state in PIM-SSM as long as receivers
are requesting receipt of that channel.

Information About SSM

The source-specific multicast (SSM) feature is an extension of IP multicast in which datagram traffic is forwarded to receivers
from only those multicast sources that the receivers have explicitly joined. For multicast groups configured for SSM, only
SSM distribution trees (no shared trees) are created.

This section describes how to configure source-specific multicast (SSM). For a complete description of the SSM commands in
this section, refer to the IP Multicast Command Reference. To locate documentation for other commands that appear in this chapter, use the command reference master index, or search
online.

SSM Components Overview

SSM is a datagram delivery
model that best supports one-to-many applications, also known as broadcast
applications. SSM is a core networking technology for the Cisco implementation
of IP multicast solutions targeted for audio and video broadcast application
environments. The
switch
supports the following components that support SSM implementation:

Protocol independent
multicast source-specific mode (PIM-SSM)

PIM-SSM is the routing
protocol that supports the implementation of SSM and is derived from PIM sparse
mode (PIM-SM).

Internet Group Management
Protocol version 3 (IGMPv3)

SSM and Internet Standard Multicast (ISM)

The current IP multicast infrastructure in the Internet and many enterprise intranets is based on the PIM-SM protocol and
Multicast Source Discovery Protocol (MSDP). These protocols have the limitations of the Internet Standard Multicast (ISM)
service model. For example, with ISM, the network must maintain knowledge about which hosts in the network are actively sending
multicast traffic.

The ISM service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host
group. The datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address
(S) and the multicast group address (G) as the IP destination address. Systems receive this traffic by becoming members of
the host group. Membership in a host group simply requires signaling the host group through IGMP version 1, 2, or 3.

In SSM, delivery of datagrams is based on (S, G) channels. In both SSM and ISM, no signaling is required to become a source.
However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific
sources. In other words, receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in
ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach
for channel subscription signaling uses IGMP and includes modes membership reports, which are supported only in IGMP version
3.

SSM IP Address Range

SSM can coexist with the ISM service by applying the SSM delivery model to a configured subset of the IP multicast group address
range. Cisco IOS software allows SSM configuration for the IP multicast address range of 224.0.0.0 through 239.255.255.255.
When an SSM range is defined, existing IP multicast receiver applications do not receive any traffic when they try to use
an address in the SSM range (unless the application is modified to use an explicit (S, G) channel subscription).

SSM Operations

An established network, in which IP multicast service is based on PIM-SM, can support SSM services. SSM can also be deployed
alone in a network without the full range of protocols required for interdomain PIM-SM (for example, MSDP, Auto-RP, or bootstrap
router [BSR]) if only SSM service is needed.

If SSM is deployed in a network already configured for PIM-SM, only the last-hop routers support SSM. Routers that are not
directly connected to receivers do not require support for SSM. In general, these not-last-hop routers must only run PIM-SM
in the SSM range and might need additional access control configuration to suppress MSDP signalling, registering, or PIM-SM
shared tree operations from occurring within the SSM range.

Use the ip pim ssm global configuration command to configure the SSM range and to enable SSM. This configuration has the following effects:

PIM operations within the SSM range of addresses change to PIM-SSM, a mode derived from PIM-SM. In this mode, only PIM (S,
G) join and prune messages are generated by the router, and no (S, G) rendezvous point tree (RPT) or (*, G) RPT messages are
generated. Incoming messages related to RPT operations are ignored or rejected, and incoming PIM register messages are immediately
answered with register-stop messages. PIM-SSM is backward-compatible with PIM-SM unless a router is a last-hop router. Therefore,
routers that are not last-hop routers can run PIM-SM for SSM groups (for example, if they do not yet support SSM).

No MSDP source-active (SA) messages within the SSM range are accepted, generated, or forwarded.

SSM Mapping

In a typical set-top box (STB) deployment, each TV channel uses one separate IP multicast group and has one active server
host sending the TV channel. A single server can send multiple TV channels, but each to a different group. In this network
environment, if a router receives an IGMPv1 or IGMPv2 membership report for a particular group, the report addresses the well-known
TV server for the TV channel associated with the multicast group.

When SSM mapping is configured, if a router receives an IGMPv1 or IGMPv2 membership report for a particular group, the router
translates this report into one or more channel memberships for the well-known sources associated with this group.

When the router receives an IGMPv1 or IGMPv2 membership report for a group, the router uses SSM mapping to determine one or
more source IP addresses for the group. SSM mapping then translates the membership report as an IGMPv3 report and continues
as if it had received an IGMPv3 report. The router then sends PIM joins and continues to be joined to these groups as long
as it continues to receive the IGMPv1 or IGMPv2 membership reports, and the SSM mapping for the group remains the same.

SSM mapping enables the last hop router to determine the source addresses either by a statically configured table on the router
or through a DNS server. When the statically configured table or the DNS mapping changes, the router leaves the current sources
associated with the joined groups.

Static SSM Mapping

With static SSM mapping, you can configure the last hop router to use a static map to determine the sources that are sending
to groups. Static SSM mapping requires that you configure ACLs to define group ranges. After configuring the ACLs to define
group ranges, you can then map the groups permitted by those ACLs to sources by using the ip igmp ssm-map static global configuration command.

You can configure static SSM mapping in smaller networks when a DNS is not needed or to locally override DNS mappings. When
configured, static SSM mappings take precedence over DNS mappings.

DNS-Based SSM Mapping

You can use DNS-based SSM mapping to configure the last hop router to perform a reverse DNS lookup to determine sources sending
to groups. When DNS-based SSM mapping is configured, the router constructs a domain name that includes the group address and
performs a reverse lookup into the DNS. The router looks up IP address resource records and uses them as the source addresses
associated with this group. SSM mapping supports up to 20 sources for each group. The router joins all sources configured
for a group.

The SSM mapping mechanism that enables the last hop router to join multiple sources for a group can provide source redundancy
for a TV broadcast. In this context, the last hop router provides redundancy using SSM mapping to simultaneously join two
video sources for the same TV channel. However, to prevent the last hop router from duplicating the video traffic, the video
sources must use a server-side switchover mechanism. One video source is active, and the other backup video source is passive.
The passive source waits until an active source failure is detected before sending the video traffic for the TV channel. Thus,
the server-side switchover mechanism ensures that only one of the servers is actively sending video traffic for the TV channel.

To look up one or more source addresses for a group that includes G1, G2, G3, and G4, you must configure these DNS records
on the DNS server:

G4.G3.G2.G1 [multicast-domain] [timeout] IN A source-address-1 IN A source-address-2 IN A source-address-n

See your DNS server documentation for more information about configuring DNS resource records.

How to Configure SSM

For a complete description of the source-specific multicast (SSM) commands in this section, see the IP Multicast Command Reference, Cisco IOS XE Release 3SE (Catalyst 3850 Switches). To locate documentation for other commands that appear in this chapter, use the command reference master index, or search
online.

Example:

Example:

Selects an
interface that is connected to hosts on which IGMPv3 can be enabled, and enters
the interface configuration mode.

The specified interface must be one of the following:

A routed port—A physical port
that has been configured as a Layer 3 port by entering the
no switchport
interface configuration command. You will also need to enable IP PIM
sparse-dense-mode on the interface, and join the interface as a statically
connected member to an IGMP static group. For a configuration example, see
Example: Interface Configuration as a Routed Port

An SVI—A VLAN interface
created by using the
interface vlan vlan-id global
configuration command. You will also need to enable IP PIM sparse-dense-mode on
the VLAN, join the VLAN as a statically connected member to an IGMP static
group, and then enable IGMP snooping on the VLAN, the IGMP static group, and
physical interface. For a configuration example, see
Example: Interface Configuration as an SVI

These interfaces must have IP
addresses assigned to them.

Step 5

ip pim {sparse-mode |
sparse-dense-mode }

Example:

Switch(config-if)# ip pim sparse-dense-mode

Enables PIM on an
interface. You must use either sparse mode or sparse-dense mode.

Step 6

ip igmp version
3

Example:

Switch(config-if)# ip igmp version 3

Enables IGMPv3 on
this interface. The default version of IGMP is set to Version 2.

Configuring Source Specific Multicast Mapping

The Source Specific Multicast (SSM) mapping feature supports SSM transition when supporting SSM on the end system is impossible
or unwanted due to administrative or technical reasons. You can use SSM mapping to leverage SSM for video delivery to legacy
STBs that do not support IGMPv3 or for applications that do not use the IGMPv3 host stack.

Example:

The ACL supplied
for
access-list defines the groups to be mapped to the
source IP address entered for the
source-address.

Note

You can configure
additional static SSM mappings. If additional SSM mappings are configured and
the router receives an IGMPv1 or IGMPv2 membership report for a group in the
SSM range, the
switch determines the source addresses
associated with the group by using each configured
ip igmp ssm-map static command. The
switch associates up to 20 sources per group.

Example:

Configuring DNS-Based SSM Mapping
(CLI)

To configure DNS-based SSM
mapping, you need to create a DNS server zone or add records to an existing
zone. If the routers that are using DNS-based SSM mapping are also using DNS
for other purposes, you should use a normally configured DNS server. If
DNS-based SSM mapping is the only DNS implementation being used on the router,
you can configure a false DNS setup with an empty root zone or a root zone that
points back to itself.

Example:

Example:

Selects an
interface on which to statically forward traffic for a multicast group using
SSM mapping, and enters interface configuration mode.

The specified interface must be one of the following:

A routed port—A physical port
that has been configured as a Layer 3 port by entering the
no switchport
interface configuration command. You will also need to enable IP PIM
sparse-dense-mode on the interface, and join the interface as a statically
connected member to an IGMP static group. For a configuration example, see
Example: Interface Configuration as a Routed Port

An SVI—A VLAN interface
created by using the
interface vlan vlan-id global
configuration command. You will also need to enable IP PIM sparse-dense-mode on
the VLAN, join the VLAN as a statically connected member to an IGMP static
group, and then enable IGMP snooping on the VLAN, the IGMP static group, and
physical interface. For a configuration example, see
Example: Interface Configuration as an SVI

These interfaces must have IP
addresses assigned to them.

Note

Static forwarding
of traffic with SSM mapping works with either DNS-based SSM mapping or
statically configured SSM mapping.

Step 4

ip igmp
static-group group-addresssource ssm-map

Example:

Switch(config-if)# ip igmp
static-group 239.1.2.1 source
ssm-map

Configures SSM
mapping to statically forward a (S, G) channel from the interface.

Use this command
if you want to statically forward SSM traffic for certain groups. Use DNS-based
SSM mapping to determine the source addresses of the channels.

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